Magnetic random access memory cell with a dual junction for ternary content addressable memory applications

ABSTRACT

A MRAM cell including a first tunnel barrier layer between a soft ferromagnetic layer having a free magnetization and a first hard ferromagnetic layer having a first storage magnetization. A second tunnel barrier layer is between the soft ferromagnetic layer and a second hard ferromagnetic layer and has a second storage magnetization. The first storage magnetization is freely orientable at a first high predetermined temperature threshold and the second storage magnetization being freely orientable at a second predetermined high temperature threshold. The first high predetermined temperature threshold is higher than the second predetermined high temperature threshold. The MRAM cell can be used as a ternary content addressable memory (TCAM) and store up to three distinct state levels. The MRAM cell has a reduced size and can be made at low cost.

FIELD

The present invention concerns a magnetic random access memory (MRAM)cell with a dual magnetic tunnel junction to be used as a ternarycontent addressable memory.

BACKGROUND

TCAM (ternary content addressable memory) is an important class ofmemory device widely used for internet networks. These elements work onmatching input data with stored address data. One feature of suchelements is that they require storing 3 distinct states 1, 0 and “don'tcare”. Normal implementations of such a device require a very largenumber of transistors to enable such functions and this leads toextremely large die sizes.

A typical implementation of a static random access memory (SRAM) TCAMcell consists of a ternary storage containing two SRAM cells whichcombines ten to twelve transistors. It also has comparison logic, whichis basically a XNOR gate using four additional pass transistors. Hencevery large cells size of fourteen to sixteen transistors, hence a costlydevice. Conventional TCAM cells are often provided as a two standardSRAM cells with four or more transistors designed to implement theexclusive-OR (EOR) function.

Unlike a RAM chip, which has simple storage cells, each individualmemory bit in a fully parallel TCAM has its own associated comparisoncircuit to detect a match between the stored data bit and the input databit. TCAM chips are thus considerably smaller in storage capacity thanregular memory chips. Additionally, match outputs from each cell in thedata word can be combined to yield a complete data word match signal.The associated additional circuitry further increases the physical sizeof the TCAM chip. Furthermore, CAM and TCAM as it is done today (usingSRAM elements) is intrinsically volatile, meaning that the data are lostwhen the power is turned off. As a result, every comparison circuitneeds being active on every clock cycle, resulting in large powerdissipation. With a large price tag, high power and intrinsicvolatility, TCAM is only used in specialized applications wheresearching speed cannot be accomplished using a less costly method.

Emerging memory technology and high-speed lookup-intensive applicationsare demanding ternary content addressable memories with large wordsizes, which suffer from lower search speeds due to large cellcapacitance.

SUMMARY

The present disclosure concerns a magnetic random access memory (MRAM)cell comprising a soft ferromagnetic layer having a magnetization thatcan be freely aligned; a first hard ferromagnetic layer having a firststorage magnetization; a first tunnel barrier layer comprised betweenthe soft ferromagnetic layer and the a first hard ferromagnetic layer; asecond hard ferromagnetic layer having a second storage magnetization;and a second tunnel barrier layer comprised between the softferromagnetic layer and the second hard ferromagnetic layer; wherein thefirst storage magnetization can be freely oriented at a first highpredetermined temperature threshold and the second storage magnetizationcan be freely oriented at a second predetermined high temperaturethreshold; the first high predetermined temperature threshold beinghigher than the second predetermined high temperature threshold.

In an embodiment, the magnetic element can further comprise a firstantiferromagnetic layer pinning the first storage magnetization belowthe first high predetermined temperature threshold, and a secondantiferromagnetic layer pinning the second storage magnetization belowthe second predetermined high temperature threshold.

In another embodiment, the first hard ferromagnetic layer can have afirst junction resistance-area product and the second hard ferromagneticlayer can have a second junction resistance-area product that issubstantially equal to the first junction resistance-area product.

The present disclosure also pertains to a method for writing to the MRAMcell comprising:

heating the magnetic element to a temperature above the firstpredetermined high temperature threshold;

applying a write magnetic field in a first direction such as to alignthe first storage magnetization and the second storage magnetization inaccordance with the write magnetic field.

In an embodiment, said write magnetic field can be applied in a firstdirection for storing a first data or in a second direction for storinga second data.

In another embodiment, the method can further comprise cooling themagnetic element (2) below the second predetermined high temperaturethreshold.

In yet another embodiment, the method can further comprise:

cooling the magnetic element to an intermediate temperature comprisedbelow the first predetermined high temperature threshold and above thesecond predetermined high temperature threshold;

applying the write magnetic field in a second direction opposed to thefirst direction, such as to align the second storage magnetization inthe second direction in accordance with the write magnetic field forstoring a third data; and

cooling the magnetic element below the second predetermined hightemperature threshold.

Also disclosed is a method for reading the MRAM cell, comprising:

measuring an initial resistance value of the magnetic element with thestored data;

providing a first search data to the sense layer and determining thematching between the first search data and the stored data; and

providing a second search data to the sense layer and determining thematching between the second search data and the stored data.

The MRAM cell disclosed herein can be used as a ternary contentaddressable memory. The MRAM cell can store three distinct state levels“1”, “0” and “X” (don't care) and be used as a matching device thusallowing an implementation as a TCAM cell with a drastically reducedcell size and cost.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood with the aid of the descriptionof an embodiment given by way of example and illustrated by the figures,in which:

FIG. 1 shows a view of a magnetic random access memory (MRAM) cellcomprising a first storage layer, a second storage layer and a senselayer according to an embodiment; and

FIGS. 2a to c illustrate the orientation of the magnetization of thefirst and second storage layer, and of the sense layer.

DETAILED DESCRIPTION OF POSSIBLE EMBODIMENTS

In an embodiment illustrated in FIG. 1, a magnetic random access memory(MRAM) cell 1 comprises a magnetic element 2 that is formed from a dualmagnetic tunnel junction comprising a first tunnel barrier layer 22having a first junction resistance-area product RA₁ and a first hardferromagnetic layer, or first storage layer 23, having a first storagemagnetization 230. The magnetic element 2 further comprises a secondtunnel barrier layer 24 having a second junction resistance-area productRA₂ and a second hard ferromagnetic layer, or second storage layer 25having a second storage magnetization 250. A soft ferromagnetic layer,or sense layer 21, having a sense magnetization 210 that can be freelyaligned is comprised between the first and second tunnel barrier layer22, 24.

In the example of FIG. 1, the magnetic element 2 further comprises afirst antiferromagnetic layer 20 and a second antiferromagnetic layer26. The first antiferromagnetic layer 20 is adapted to exchange couplethe first storage layer 23 such that the first storage magnetization 230can be freely oriented at a first high temperature threshold Tw1, andpinned below this temperature. The second antiferromagnetic layer 26 isadapted to exchange couple the second storage layer 25 such that thesecond storage magnetization 250 can be freely oriented at a second hightemperature threshold Tw2, and pinned below this temperature. In anembodiment, the first predetermined high temperature threshold Tw1 isgreater than the second predetermined high temperature threshold Tw2.

In an illustrative example, the first storage layer 23 can be made froma NiFe/CoFeB-based alloy and the first antiferromagnetic layer 20 can bemade from an IrMn-based alloy. The second storage layer 25 can be madefrom a CoFeB/NiFe-based alloy and the second antiferromagnetic layer 26can be made from a FeMn-based alloy. The sense layer 21 is preferablymade from a CoFeB-based alloy.

In an embodiment, the first junction resistance-area product RA₁ issubstantially equal to the second junction resistance-area product RA₂.A first tunnel magneto resistance TMR₁ of the first tunnel barrier layer22, first storage layer 23 and the sense layer 21 will then besubstantially the same as a second tunnel magneto resistance TMR₂ of thesecond tunnel barrier layer 24, second storage layer 25 and the senselayer 21. The first and second tunnel barrier layer 22, 24 arepreferably made from MgO with where both the first and second junctionresistance-area products RA₁, RA₂ are substantially equal to 20 ohm μm².

The MRAM cell 1 can further comprises a current line (not represented)in electrical communication with one end of the magnetic element 2 and aselection transistor (also not represented) in electrical communicationwith the other end of the magnetic element 2.

In an embodiment, the MRAM cell 1 is written using a thermally assistedwrite operation. More particularly, providing a first write data to theMRAM cell 1 comprises the steps of:

heating the magnetic element 2 to/at a temperature above the firstpredetermined high temperature threshold Tw1, such as to free the firstand second storage magnetizations 230, 250;

applying a write magnetic field in a first direction such as to alignboth the first storage magnetization 230 and the second storagemagnetization 250 in that first direction in accordance with the writemagnetic field; and

cooling the magnetic element 2 to a temperature that is below the secondpredetermined high temperature threshold Tw2 such that the first andsecond storage magnetization 230, 250 are pinned in the written state bythe first and second antiferromagnetic layer 20, 26, respectively.

After cooling and in the absence of the write magnetic field, the senselayer 21 is in the equilibrium state and its sense magnetization 210 isoriented antiparallel to the first and second storage magnetization 230,250. A first stored data “0” thus corresponds to the magnetic element 2having a first state level. The orientation of the first storagemagnetization 230, the second storage magnetization 250 and of the sensemagnetization 210 are illustrated in FIGS. 2a to c . In FIGS. 2a to cthe first and second antiferromagnetic layers 20, 26 are notrepresented. More particularly, FIG. 2a represents the orientation ofstorage magnetizations 230, 250 and of the sense magnetization 210 forthe first stored data “0”.

In another embodiment, a second write data can be provided to themagnetic element 1 by performing the heating step described above. Thewrite magnetic field is then applied a second direction that is opposedto the first direction, such as to align both the first storagemagnetization 230 and the second storage magnetization 250 in the seconddirection. After cooling to a temperature below the second predeterminedhigh temperature threshold Tw2, and in the absence of the write magneticfield (equilibrium), the sense magnetization 210 is orientedantiparallel to the first and second storage magnetization 230, 250. Thesecond stored data “1” thus corresponds to the magnetic element 2 havinga second state level (see FIG. 2b ).

In another embodiment, providing a third write data is performed by:

heating the magnetic element 2 to/at a temperature above the firstpredetermined high temperature threshold Tw1, such as to free the firstand second storage magnetizations 230, 250;

applying the write magnetic field in the first direction such as toalign both the first storage magnetization 230 and the second storagemagnetization 250 in the first direction in accordance with the writemagnetic field;

cooling the magnetic element 2 to/at an intermediate temperature that iscomprised below the first predetermined high temperature threshold Tw1and above the second predetermined high temperature threshold Tw2 suchas to pin the first storage magnetization 230 by the firstantiferromagnetic layer 20 such that the second storage magnetization250 can be freely oriented;

applying the write magnetic field in the second direction such as toalign the second storage magnetization 250 in the second direction inaccordance with the write magnetic field; and

cooling the magnetic element 2 to a temperature below the secondpredetermined high temperature threshold Tw2 such that the first andsecond storage magnetization 230, 250 are pinned in the written state bythe first and second antiferromagnetic layer 20, 26, respectively.

In this latter configuration, the sense magnetization 210 can beoriented either in the first or second direction, i.e., parallel orantiparallel to the first and second storage magnetization 230, 250. Thethird stored data “X” thus corresponds to the magnetic element 2 havinga third intermediate state level (see FIG. 2c ).

Heating the magnetic element 2 can be performed by passing a heatingcurrent (not shown) in the magnetic element 2, via the current line.Cooling the magnetic element 2 to the intermediate temperature thresholdcan thus be performed by reducing the heating current intensity, andcooling the magnetic element 2 to the low temperature threshold can beachieved by blocking the heating current. Applying the write magneticfield can be performed by passing a field current (not shown) in thecurrent line.

According to an embodiment, a read operation of the written MRAM cell 1comprises:

measuring an initial resistance value R_(o) of the magnetic element 2with the stored data;

providing a first search data “0” to the sense layer 21 and determiningthe matching between the first search data and the stored data; and

providing a second search data “1” to the sense layer 21 and determiningthe matching between the second search data and the stored data.

Measuring the initial resistance R_(o) is performed by passing a readcurrent in the magnetic element 2 in the absence of external magneticfield (zero field). Providing the first and second search data “0”, “1”comprises applying the read magnetic field in a first and seconddirection, respectively, such as to orient the sense magnetization 210in accordance with the read magnetic field direction. Determining thematching between the first and second search data and the stored datacomprises measuring a first read resistance R₁ and a second readresistance R₂ by passing the read current in the magnetic element 2 whenthe magnetic field is applied in the first and second direction,respectively.

The read operation disclosed herein is a self-referenced based readoperation in the sense that the resistance of the magnetic element 2 ismeasured for the first and second search data “0”, “1” (first and secondread resistance) and the use of the reference cell is not required. Suchself-referenced based read operation has also been disclosed in Europeanpatent application EP2276034 by the present applicant. Moreover, theread operation disclosed herein comprises determining the matchingbetween the first and second search data and the stored data, instead ofsimply reading a stored value “0” or “1” as in a conventional readoperation.

In the case of the first stored data “0” the initial resistance R_(o)has a high value (R_(max1)+R_(max2)) that is determined by a first highresistance (R_(max1)) due to the antiparallel orientation of the sensemagnetization 210 with the first storage magnetization 230 and a secondhigh resistance (R_(max2)) due to the antiparallel orientation of thesense magnetization 210 with the second storage magnetization 250 (seeTable I). Providing the first search data “0” orients the sensemagnetization 210 antiparallel to the first and second storagemagnetization 230, 250, and the measured value of the first readresistance R₁ is high (R_(max1)+R_(max2)). Providing the second searchdata “1” orients the sense magnetization 210 parallel to the first andsecond storage magnetization 230, 250, and the measured value of thesecond read resistance R₂ is low (R_(min1)+R_(min2)), where R_(min1) isa first low resistance due to the parallel orientation of the sensemagnetization 210 with the first storage magnetization 230, and R_(min2)is a second low resistance due to the parallel orientation of the sensemagnetization 210 with the second storage magnetization 230. Here, thedifference between R₂ and R₁ corresponds toΔR=(R_(max1)+R_(max2))−(R_(min1)+R_(min2)).

In the case of the second stored data “1” the initial resistance R_(o)has a high value (R_(max1)+R_(max2)) Providing the first search data “0”orients the sense magnetization 210 parallel to the first and secondstorage magnetization 230, 250 and the measured value of the first readresistance R₁ is low (R_(min1)+R_(min2)). Providing the second searchdata “1” orients the sense magnetization 210 antiparallel to the firstand second storage magnetization 230, 250 and the measured value of thesecond read resistance R₂ is high (R_(max1)+R_(max2)).

In the case of the third stored data “X” the initial resistance R_(o)has an intermediate value (R_(min1)+R_(max2)). Providing the firstsearch data “0” orients the sense magnetization 210 parallel to thefirst storage magnetization 230 and antiparallel to the and secondstorage magnetization 250. Providing the second search data “1” orientsthe sense magnetization 210 antiparallel to the first storagemagnetization 230 and parallel to the second storage magnetization 250.The measured value for both the first and second read resistance R₁, R₂corresponds to an intermediate value (R_(min1)+R_(max2)). The same valueof the first and second read resistance R₁, R₂ for the two search data“0” and “1” corresponds to a “don't care” state level of the MRAM cell1, since the output (read resistance) is insensitive to the input state(search data). Table 1 reports the different resistance values measuredfor the initial resistance R_(o) and first and second read resistanceR₁, R₂.

TABLE 1 stored data stored data stored data Ro (zero field) R_(max1) +R_(max2) R_(max1) + R_(max2) R_(min1) + R_(max2) R₁ (search dataR_(max1) + R_(max2) R_(min1) + R_(min2) R_(min1) + R_(max2) R₂ (searchdata R_(min1) + R_(min2) R_(max1) + R_(max2) R_(min1) + R_(max2)

The MRAM cell 1 disclosed herein can thus be used as a ternary contentaddressable memory. The MRAM cell 1 can store three distinct statelevels “1”, “0” and “X” (don't care) and be used as a matching devicethus allowing an implementation as a TCAM cell with a drasticallyreduced cell size and cost. Table 2 reports the matching between thefirst and second search data and the stored data. More particularly,determining the matching further comprises comparing the measured firstand second read resistance (R₁, R₂) to the initial resistance value(R_(o)). A match corresponds to no change between the initial resistancevalue R_(o) and the value of the first and second read resistance R₁,R₂.

TABLE 2 stored data stored data stored data search data No change Nochange No change “X” (zero (match) (match) (match) search data No changeR₂ decreases by No change search data R₂ decreases by No change Nochange

An additional advantage of the MRAM cell 1 is that, during the writeoperation, both the first and second tunnel barrier layer 22, 24contribute in heating the magnetic element 2 at the first and secondpredetermined high temperature threshold Tw1, Tw2. Consequently, thepower required when passing the heating current for heating the magneticelement 2 can be reduced by a factor of about √2 as compared to thepower required in a conventional magnetic tunnel junction comprising asingle tunnel barrier layer. This results in an enhanced endurance ofthe MRAM cell 1 to voltage cycling during the write operation.

In addition, the MRAM cell 1 disclosed herein allows for comparing a“don't-care” search data. This can be handled by a second read at zerofield. Clearly this leads to no resistance change and thus results in amatch irrespective or the written state (search data “X”).

REFERENCE NUMBERS AND SYMBOLS

-   1 MRAM cell-   2 magnetic element-   20 first antiferromagnetic layer-   21 sense layer-   22 first tunnel barrier layer-   23 first storage layer-   24 second tunnel barrier-   25 second storage layer-   26 second antiferromagnetic layer-   210 sense magnetization-   230 first storage magnetization-   250 second storage magnetization-   ΔR difference between R₁ and R₂-   RA₁ first junction resistance-area product-   RA₂ second junction resistance-area product-   R_(max1) first high resistance value-   R_(max2) second high resistance value-   R_(min1) first low resistance value-   R_(min2) second low resistance value-   R_(o) initial resistance-   R₁ first read resistance-   R₂ second read resistance-   TMR₁ first tunnel magneto resistance-   TMR₂ second tunnel magneto resistance-   Tw1 first predetermined high temperature threshold-   Tw2 second predetermined high temperature

1. A method for reading a MRAM cell including a magnetic elementcomprising a soft ferromagnetic layer having a magnetization that can befreely aligned; a first hard ferromagnetic layer having a first storagemagnetization; a first tunnel barrier layer comprised between the softferromagnetic layer and the a first hard ferromagnetic layer; a secondhard ferromagnetic layer having a second storage magnetization; and asecond tunnel barrier layer comprised between the soft ferromagneticlayer and the second hard ferromagnetic layer; the first storagemagnetization being be freely orientable at a first high predeterminedtemperature threshold and the second storage magnetization being freelyorientable at a second predetermined high temperature threshold; thefirst high predetermined temperature threshold being higher than thesecond predetermined high temperature threshold; the method comprising:measuring an initial resistance value of the magnetic element with thestored data; providing a first search data to the sense layer anddetermining the matching between the first search data and the storeddata; and providing a second search data to the sense layer anddetermining the matching between the second search data and the storeddata.
 2. The method according to claim 1, wherein measuring the initialresistance comprises passing a read current in the magnetic element inthe absence of external magnetic field.
 3. The method according to claim1, wherein providing the first search data comprises applying the readmagnetic field in a first direction and providing the second search datacomprises applying the read magnetic field in a second direction; suchas to orient the sense magnetization in the first and second direction,respectively.
 4. The method according to claim 1, wherein determiningthe matching between the first and second search data and the storeddata comprises measuring a first read resistance and measuring a secondread resistance by passing the read current in the magnetic element withthe magnetic field being applied in the first and second direction,respectively.
 5. The method according to claim 1, wherein determiningthe matching further comprises comparing the measured first and secondread resistance to the initial resistance value.